Glaciers are powerful forces that shape the Earth’s surface, leaving behind distinctive landforms. These slow-moving masses of ice grind and sculpt the underlying rock, carrying vast quantities of sediment. When the ice melts, this material is released and redistributed by meltwater streams, creating widespread deposits. One of the most expansive features formed by this process is the outwash plain, a broad, flat expanse built from glaciofluvial sediments.
Defining the Outwash Plain
An outwash plain is a broad, gently sloped sheet of sediment deposited by meltwater streams flowing away from a glacier’s terminus. Geologists refer to the environment directly in front of a glacier as the “proglacial” zone, where these plains develop. The term sandur (plural sandar) is the Icelandic name for this landform.
The material that makes up the plain is called stratified drift. This indicates that the sediment has been sorted and deposited in layers by flowing water, unlike the unsorted material (till) deposited directly by the ice. The sorting occurs because meltwater streams lose energy as they spread out, dropping heavier, coarser sediments first and carrying finer particles further away.
The Process of Formation
The creation of an outwash plain is a dynamic process driven by the immense volume and energy of glacial meltwater. As a glacier retreats or experiences seasonal melting, water flows out from the ice front, carrying a massive load of sediment previously eroded and transported by the glacier.
The high-velocity streams carry everything from fine silt and sand to large gravel and cobbles. Once the water emerges from the confines of the ice or valley, it spreads out across the flatter proglacial area, causing a rapid decrease in velocity. This loss of energy forces the water to drop its sediment load, with the largest particles settling closest to the glacial terminus.
This continuous process of sediment transport and deposition builds up a vast, sheet-like plain. The meltwater streams flowing across this surface are often characterized by a braided pattern, where the river splits into numerous small, interwoven channels. This braiding results from the high sediment load and variable discharge, which constantly forces the streams to deposit material and change course.
Geological Characteristics and Composition
Outwash plains possess distinct physical characteristics reflecting their formation by water-sorting processes. The composition is dominated by well-sorted layers of sand and gravel, deposited in alternating units of different grain sizes, known as cross-bedding. This layered structure, the stratified drift, distinguishes the outwash plain from the unsorted till found in moraines.
The topography of the plain is typically flat, extending for many kilometers, and slopes gently away from the former glacial margin. The coarse, permeable nature of the sand and gravel results in highly porous soils. The surface is often punctuated by depressions called kettles, which form when isolated blocks of ice are buried by accumulating sediment.
The buried ice blocks are protected by the sediment, acting as an insulating layer. When the ice eventually melts, the overlying sediment collapses, creating circular or irregular depressions. If these depressions fill with water, they are known as kettle lakes, which are common features across formerly glaciated landscapes.
Global Examples and Regional Significance
Outwash plains are found globally in areas once covered by large continental ice sheets or in front of contemporary mountain glaciers. Iceland is home to some of the most extensive and active examples, referred to as sandar, due to high rates of glacial melt accelerated by geothermal activity. The Skeiðarársandur in southeastern Iceland is one of the largest such plains, covering approximately 1,300 square kilometers.
Ancient outwash plains from past ice ages, such as the Wisconsin Glaciation, cover vast areas. For example, deposits can be traced hundreds of kilometers from the former ice margin in North America. The coarse, well-sorted nature of the sand and gravel makes these plains highly significant for human use.
The excellent porosity and permeability of the stratified drift allow the plains to serve as major aquifers for freshwater storage and supply. The generally flat topography and relative abundance of water also make them suitable for certain types of agriculture and for the development of infrastructure, such as roads and settlements. The materials themselves are often mined as a source of commercial sand and gravel for construction purposes.